Magnetic memories, particularly magnetic random access memories (MRAMs), have drawn increasing interest due to their potential for high read/write speed, excellent endurance, non-volatility and low power consumption during operation. An MRAM can store information utilizing magnetic materials as an information recording medium. One type of MRAM is a spin transfer torque random access memory (STT-RAM). STT-RAM utilizes magnetic elements written at least in part by a current driven through the magnetic element.
For example, FIG. 1 depicts a conventional magnetic tunneling junction (MTJ) 10 as it may be used in a conventional STT-RAM. The conventional MTJ 10 typically resides on a bottom contact 11, uses conventional seed layer(s) 12 and includes a conventional antiferromagnetic (AFM) layer 14, a conventional pinned layer 16, a conventional tunneling barrier layer 18, a conventional free layer 20, and a conventional capping layer 22. Also shown is top contact 24.
Conventional contacts 11 and 24 are used in driving the current in a current-perpendicular-to-plane (CPP) direction, or along the z-axis as shown in FIG. 1. The conventional tunneling barrier layer 18 is nonmagnetic and is, for example, a thin insulator such as MgO. The conventional seed layer(s) 12 are typically utilized to aid in the growth of subsequent layers, such as the AFM layer 14, having a desired crystal structure. Direct exposure of the conventional free layer 20 to the top contact 24 may result in a disordered interface, dead magnetic regions and enhanced damping. Consequently, the conventional capping layer 22 is provided directly on the free layer 20, prior to deposition of the top contact 24. This conventional cap acts as a diffusion block and improves the surface quality of the conventional free layer 24.
The conventional pinned layer 16 and the conventional free layer 20 are magnetic. The magnetization 17 of the conventional pinned layer 16 is fixed, or pinned, in a particular direction, typically by an exchange-bias interaction with the AFM layer 14. Although depicted as a simple (single) layer, the conventional pinned layer 16 may include multiple layers. For example, the conventional pinned layer 16 may be a synthetic antiferromagnetic (SAF) layer including magnetic layers antiferromagnetically or ferromagnetically coupled through thin conductive layers, such as Ru. In such a SAF, multiple magnetic layers interleaved with a thin layer of Ru may be used. Further, other versions of the conventional MTJ 10 might include an additional pinned layer (not shown) separated from the free layer 20 by an additional nonmagnetic barrier or conductive layer (not shown).
The conventional free layer 20 has a changeable magnetization 21. Although depicted as a simple layer, the conventional free layer 20 may also include multiple layers. For example, the conventional free layer 20 may be a synthetic layer including magnetic layers antiferromagnetically or ferromagnetically coupled through thin conductive layers, such as Ru. Although shown as in-plane, the magnetization 21 of the conventional free layer 20 may have a perpendicular anisotropy. For example, a perpendicular anisotropy may be induced in the conventional free layer 20. If the out-of-plane demagnetization energy exceeds the energy associated with the perpendicular anisotropy (perpendicular anisotropy energy), the perpendicular anisotropy may be termed a partial perpendicular magnetic anisotropy (PPMA). Thus, the magnetization 21 remains in plane despite having a perpendicular-to-plane anisotropy. If the out-of-plane demagnetization energy is less than the perpendicular anisotropy energy, then the magnetization of the free layer 20 would be out of plane (e.g. in the z direction in FIG. 1). A perpendicular magnetization 21 may be desirable for a variety of reasons, such as reducing the current density required to write to the free layer 20.
The conventional MTJ 10 is also required to be thermally stable for use in STT-RAM. During periods of latency, thermal fluctuations allow the magnetic moments within the conventional free layer 20 to oscillate and/or precess. These thermal fluctuations may result in the reversal of the magnetization 21 of the conventional free layer 20, making the conventional MTJ 10 unstable. In order to provide thermal stability against such fluctuations, the energy barrier separating oppositely oriented magnetization states in the free layer 20 is desired to be of sufficient magnitude. Typically, this is achieved at least in part by ensuring that the conventional free layer 20 has a sufficient volume. In addition, the free layer 20 generally has a number of anisotropies associated with it. The out-of-plane demagnetization energy relates to the shape anisotropy associated with the thin film anisotropy and generally confines the magnetization of the free layer 20 in plane. In the conventional MTJ 10 shown in FIG. 1, the conventional free layer 20 may have a shape anisotropy, allowing the free layer magnetization 21 to be stable along the x-axis as shown in FIG. 1. Further, there may be additional anisotropies, for example associated with the crystal structure of the conventional free layer 20.
Although the conventional magnetic element 10 may be written using spin transfer and used in an STT-RAM, there are drawbacks. In general, it is desirable to scale to higher memory densities and, therefore, smaller sizes of the conventional magnetic element 10. Magnetic elements 10 may have barriers to such scaling regardless of whether free layer magnetizations 21 are in-plane or perpendicular-to-plane. In particular, in the case that the magnetization 21 of the conventional free layer is in-plane, the current density required to write to the conventional magnetic element 10 may still be relatively high, while the magnetoresistance may still be lower than desired. For conventional magnetic elements in which the magnetization 21 of the conventional free layer 20 is perpendicular to plane, there may also be barriers to scaling. Typically, such magnetic elements 10 use high deposition temperatures, which may be inconsistent with preservation of other layers in the stack, such as the conventional barrier layer 18. The materials used in such a multilayer may also be limited if, as is generally desired, crystalline MgO is to be used as the conventional barrier layer 18. Finally, such magnetic elements may have asymmetric RH loops, which is undesirable for use in a magnetic memory.
Accordingly, what is needed is a method and system that may improve the scaling of the spin transfer torque based memories. The method and system described herein address such a need.